1. Field of the Invention
The present invention relates to an indicating instrument having a stepping motor and, more particularly, relates to an indicating instrument such as a speedometer for a vehicle which has a stepping motor as a movement.
2. Description of the Related Art
Conventionally, a two-phase PM-type stepping motor, in which a magnet rotor is movably installed in a couple of stators, is adopted in an indicating instrument for a vehicle. In such an indicating instrument, sine (SIN) wave current and cosine (COS) wave current are respectively supplied to the stator coils at the same phase so as to rotate the magnet rotor according to a magnetic field composed of magnetic fields generated by the respective currents, thereby driving the indicator.
The indicator is kept staying at the zero position until the instrument is shipped.
However, if mechanical vibration or a shock is applied to the indicating instrument before it is shipped before being mounted on a vehicle, the magnet rotor may rotate to separate the indicator away from the zero position. In addition, even if such shock is applied to the indicator instrument after it is mounted on a vehicle, the indicator may be separated from the zero position.
In order to solve the above problem, JP-A-6-38593 proposes to provide a stopper near the zero position of a dial plate and engage the indicator with the stopper by supplying the sine wave and cosine wave currents to the respective stator coils to rotate the magnet rotor in the direction of the indicator returning to the zero position.
However, in the indicator instrument disclosed in the above publication, the sine and cosine wave currents keep flowing to maintain the magnetic field even after the indicator engages the stopper. As a result, the magnet rotor stands still with the indicator in engagement with the stopper, while the magnetic field generated by the stator rotates. Accordingly, the attracting force and the repulsion force are alternately generated between the stator and the rotor, and the indicator vibrates in a manner that it leaves or engages the stopper. Such vibration deteriorates the visibility of the indicating instrument and makes a noise.